Industrial users of induction motors are interested in finding reliable procedures for rotor fault detection and diagnostics, since a sudden motor failure may be very damaging or catastrophic in an industrial system, in which the electric motor is a prime mover. These motor failures can force expensive shutdowns of factory production and consequently reduce productivity. Hence, maintenance schedules are provided to proactively reduce or prevent these failures. Nevertheless, the probability of a sudden motor failure cannot be entirely ruled out. Moreover, increasing the frequency of scheduled maintenance increases the cost and decreases the productivity of a system. Accordingly, an online fault diagnostic system becomes a valuable tool to increase industrial productivity and process reliability.
Accordingly, different methods have been investigated and reported to detect rotor faults in induction motors that either make use of off-line detectors or require installing an additional tool in an induction motor. These methods require speed (or slip) measurement (or determination) or require knowledge of other motor parameters. For example, U.S. Pat. No. 4,808,932 discloses a broken bar detector that requires the installation of a flux coil wound on one of the stator teeth of an induction motor. This requires motor disassembly, unless the flux coil is installed at the time of manufacturing or rewinding. Moreover, this special arrangement increases the cost and decreases the reliability of the diagnostic system.
U.S. Pat. No. 5,049,815 discloses a broken bar detector that requires an accurate determination of the motor slip frequency. Determining slip frequency, from monitoring the current drawn by a motor in order to track the side band components as an index for fault diagnostic, reduces the reliability of the diagnostic system particularly for induction motor-drive systems, where the supply frequency varies by a closed loop control-drive system.
U.S. Pat. No. 6,650,122 discloses a rotor analyzer which requires an electromagnetic coil. Basically this analyzer is not an on-line detector, but instead checks a rotor that is not installed within a motor's stator housing.